Gravel roads, gravel shoulders, gravel parking lots, all exemplify the use of gravel in economically facilitating both vehicular and pedestrian traffic over defined areas and routes. For various reasons, the use of gravel in these applications can be very much more attractive than the alternatives afforded by asphalt and concrete paving.
The use of gravel, however, is not without its own complications. Uneven compaction over the travelled surface can lead to the formation of wheel ruts, while turning, accelerating or decelerating traffic can lead to local phenomenon such as potholes and corrugations; and, the aggregate material will itself tend to undergo a sorting, by particle size, attributable to differential compaction effects, the results of all of which are less than attractive. In some cases, these effects represent safety hazards.
One solution to the degradation of such gravel emplacements has been to simply add additional gravel over the surface, as needed. Interestingly, there is only a limited consolidation between the added material and the underlying surface, which tends to leave the upper, relatively unconsolidated material in an easily disturbed layer from which aggregate material tends to be rapidly displaced. Moreover, commercial sources of aggregate depend of naturally occurring gravel deposits, and like most natural resources, their numbers are becoming depleted. Although the existence of many more marginal deposits stave off any likelihood of critical shortages of supply, the costs of harvesting a more marginal deposit is inherently higher, and this is reflected in higher gravel costs.
As a consequence, the historical practice most often adopted to remediate gravel roads, parking lots and the like, have entailed the use of large, powerful road graders, equipped with scarifiers. A grader is, by definition, a machine with a centrally located blade that can be angled to either side. Typically, graders have a reinforced tubular or box-beam "Y" frame supporting the engine at the rear of the vehicle, between the arms of the "Y". Drive wheels, usually arranged in tandem, are positioned below the engine and transmission, while steering wheels are arranged on an axel system at the point of the "Y" frame. The major attachments for the grader are secured in downwardly hung relation from the overhead portions of the frame, and are pulled by a drawbar reaching back from the front of the frame. These attachments include the blade and the scarifier.
More specifically, the blade is arranged on a toothed ring gear called the "circle", on which the attachments can be rotated. An arm-type attachment between the "circle" and the frame allow the circle to be controllably lifted, lowered, offset to either side, or even to be placed into a vertical configuration. The scarifier is typically position in front of the blade, and is carried on a pair of arms that reach back from the front end of the graders frame. The scarifier can be raised and lowered to regulate the depth of penetration, relative to the bottom of the graders tires. The number of teeth used on the scarifier is dependent on the hardness of the surface being worked.
Attempts at smaller scale scarifiers have been unsuccessful from a performance point of view, and have not enjoyed commercial acceptance. One such attempt took the form of a towed "box scarifier". This consisted of a scarifier and a plough blade arranged at opposed ends of a frame which a had a box shaped plan. The device was dragged along by a vehicle with the intention that the scarifier would turn up the underlying aggregate and the plough blade would evenly redistribute it. Unfortunately the device was neither heavy enough to scarify properly, nor did it afford the control necessary to evenly redistribute even such material as was dislodged by the scarifier teeth. The problem is similar to that encountered with very early attempts to use towed road construction rippers, the use of which has now apparently been abandoned. Towed rippers too, proved to be unsatisfactory do to poor penetration. If sufficient weight was added to the towed ripper to insure effective penetration, the ripper became too heavy for any but the largest of commercial tractor vehicles. These towed rippers were also rather unwieldy, and hard to manoeuvre. Lastly, such rippers were not grading devices, and are simply a battery of teeth mounted on a wheeled vehicle.
Towed graders were also known. These were produced in an attempt to deal with a number of grader-related problems, but are now considered outmoded. Their use required two skilled operators, which was an offset to the presumed advantage of reduced capital and maintenance costs. They were in any case, found to be hard to manoeuvre even relative to motorized graders, and were never known to be used in scarifying operations. There is the further problem that the tow vehicle tends to compact the material that the towed grader was intended levelingly redistribute.
Accordingly, the only known commercially viable practice continues to rely on the use of motorized graders. Graders provide sufficient weight and power to force the scarifier teeth into the ground and drag it along, together with the control necessary to position a grader blade for proper redistribution of the dislodged materials.
The problems that are and have always been immediately apparent in connection with the use of graders for this purpose continue to be a problem, however. These include a requirement for a highly skilled operator. Moreover, graders are not highly manoeuvrable, a problem which is a function of the very size and weight heretofore thought necessary to achieve scarifying/grading operations. Moreover, the economics of grader operation are very sensitive. For example, according to "HEAVY CONSTRUCTION--Equipment and Methods"--by Stuart Wood Jr., points out that straight line grading patterns that cover less than 1000 linear feet, are economically inefficient because of the operating/time costs associated with turning the grader around. Even were the economics otherwise, the large size of graders make them difficult or even impossible to manoeuvre in the manner necessary to service the close quarters that characterize many gravel emplacements. The travelling costs of grader equipment are very high, and some jobs are often not done at all particularly if they are in geographically isolated areas.